Fix blurry preview on HiDPI displays

THe captured pixmap should always have devicePixelRatio one, it only
needs to be changed when it is actually drawn in the preview widget.

Also fixes a regression from e4c2e564
("Replace KScreen by QScreen for current window grab"), now the current
screen grab mode will again capture the full screen on HiDPI screens.

Tested with: QT_SCALE_FACTOR=2 src/spectacle

REVIEW: 129189